Squirrels - friend or family?

Bill and I have been observing some squirrels in our backyard over the past two days.  Bill saw these two at the feeder (I missed that) and then, a bit later, I saw them playing in the tree just outside the gate.  One is a black (or very deep brown) squirrel and the other is grey with a lighter belly.  We wondered if two different color squirrels could be family.  If not, are they friends playing, or enemies competing for food, with one trying to chase the other away.  My vote is that they are enjoying a good play session.  Friend or family, I have no idea. 

Here, the grey one is chasing the black one across the garage roof.  I only caught the tail of the black one.
We watched the black one try for some time to get up on the tray but he never did make it.  That surprised both of us, as squirrels' athleticism is well known.  Maybe he was too heavy, or maybe they need wood or some other material for their claws to grip.
Here, I'm quite sure he was watching us watch him through the window.
Today, we saw this other little black fellow.  I first saw him huddled in a corner by the back door.  Later, Bill saw him eating out of the tray, so unlike the larger one, he was able to make it up at least once.  We saw him try again later, but this time, after three attempts, he gave up.  We both wondered if he is just a small, sickly squirrel, or whether he is possibly the offspring of the larger black one shown above.  We are both concerned for him, and hoping the seeds and nuts we've put out might help him to gain some weight and grow some healthier fur.  He appears to have good energy, as he went back and forth several times from the tray to my neighbor's garden, I think burying his cache there.  I've included several pictures of him, as I find him both endearing and pathetic at the same time.  I could tell Bill also has a soft spot in his heart for this little one.